1. FIELD OF THE INVENTION
This invention relates to engine retarders of the compression relief type used in connection with supercharged internal combustion engines equipped with an air-to-air charge cooling system operated by a turbofan. More particularly the invention relates to a solenoid activated valve designed to deactivate the turbofan when the compression relief engine retarder is in operation.
2. PRIOR ART
For many years it has been recognized that vehicles, and particularly trucks, equipped with internal combustion engines of the Otto or Diesel type should be provided with some form of engine retarder in addition to the usual wheel brakes. The reason for this is that the momentum of a heavily loaded vehicle descending a long grade is so great that the vehicle wheel brakes may easily be overheated, resulting in "brake fade", and, in some cases, may be destroyed.
Various types of engine retarders have been developed including hydrokinetic retarders, electrical retarders, compression relief engine brakes and exhaust brakes. Each of these types of engine retarder has been described in the book "Retarders For Commercial Vehicles" published by Mechanical Engineering Publications, Ltd. (London, 1974).
The present invention relates particularly to engine retarders of the compression relief type in which an engine is converted temporarily into an air compressor by opening the exhaust valves at or near the end of the compression stroke of the engine. By so opening the exhaust valves out of sequence, the energy required to compress air in the cylinder is released through the exhaust system instead of being recovered during the power stroke of the engine. This energy, known as the retarding horsepower, may be a substantial portion of the power ordinarily developed by the engine during a fueling mode of operation and is effective as a supplemental braking system. The retarding horsepower developed by a typical engine, such as the Mack 676 engine fitted with a Jacobs engine brake, may be in the vicinity of 200 horsepower at an engine speed of 2100 RPM. The Jacobs engine brake is described in detail at pp. 23-30 of the publication "Retarders For Commercial Vehicles" referred to above and is described generally in Cummins U.S. Pat. No. 3,220,392.
Frequently, in order to increase the power of an internal compression engine, superchargers have been installed which increase the pressure, and therefore the mass, of the air charged into the engine. With a greater mass of air in the cylinder, more fuel can be combusted and higher mean effective pressures (M.E.P.) obtained, thus increasing the power output of the engine. While various types of superchargers have been utilized, a common and popular form of supercharger is a turbine driven centrifugal compressor, frequently referred to as a turbocharger. In the usual turbocharger, the turbine is driven by the exhaust gases leaving the exhaust manifold.
Bearing in mind that the volume of air compressed by the turbocharger varies directly with its absolute temperature, the engine operation can be improved by cooling the compressed air prior to admission into the inlet manifold. For this purpose an air-to-air intercooler driven by a turbofan has been employed. In the air-to-air intercooler, ambient air is passed in heat exchange relationship with the compressed air from the turbocharger thereby cooling the compressed air prior to its admission into the engine. In order to provide effective heat exchange in the intercooler, a small turbofan is provided. The turbofan, also known as a tip turbine fan, has turbine blades at the tip or circumferential edge of the fan. The fan may be driven by bleeding or bypassing a portion of the air compressed in the turbocharger through the turbine blades of the tip turbine fan. The fan then drives ambient air past the duct containing air compressed by the turbocharger thereby cooling the compressed air.
The advantages obtained by the use of an intercooler include the ability of the engine to burn more fuel and thus to develop more power while operating at lower exhaust temperatures which reduces the mechanical and thermal load on the engine and its various parts.